hmm... it all depends on the situation. if you were out of school and dependant only on yourself there would be no problems. after all, a lot of 30 year old women are married to 45 year old men and no one thinks twice about this.
the issue you would have to address here is control. if you were actually in a relationship with this person, you do not have enough life experience to judge situations as well as they do, and you would be easily led.
don't completely reject this, i actually speak from experience. i was 15 and went out with someone who was 30. i was young and naive, but bloody lucky. he was a nice guy who was only looking out for my needs and wants and allowed me to make decisions. [and i'm aware that at least someone will say 'ewww thats gross, how could you go out with someone twice your age' but i do not, repeat - do not care]
also there is the issue on how your friends and family would take it. it is a bit of a taboo issue nowadays. my family did not take to my relationship lightly. in fact, they rang the police. but i won't go into that.
with these kinds of things you just have to be careful